Pattern recognition receptors confer plant resistance to pathogen infection by recognizing the conserved pathogen-associated molecular patterns. The cell surface receptor chitin elicitor receptor kinase 1 of Arabidopsis (AtCERK1) directly binds chitin through its lysine motif (LysM)-containing ectodomain (AtCERK1-ECD) to activate immune responses. The crystal structure that we solved of an AtCERK1-ECD complexed with a chitin pentamer reveals that their interaction is primarily mediated by a LysM and three chitin residues. By acting as a bivalent ligand, a chitin octamer induces AtCERK1-ECD dimerization that is inhibited by shorter chitin oligomers. A mutation attenuating chitin-induced AtCERK1-ECD dimerization or formation of nonproductive AtCERK1 dimer by overexpression of AtCERK1-ECD compromises AtCERK1-mediated signaling in plant cells. Together, our data support the notion that chitin-induced AtCERK1 dimerization is critical for its activation.

Foreshock bubbles (FBs), transient ion foreshock phenomena formed from highly concentrated suprathermal foreshock ions upstream of solar wind discontinuities, produce intense disturbances in the magnetosphere‐ionosphere system and can accelerate particles to even higher energies. Rotational discontinuities are known to drive FBs. Tangential discontinuities (TDs), however, have not previously been considered as drivers of FBs because they have no normal magnetic field component, preventing access of field‐aligned particles upstream. However, given that suprathermal foreshock ions have gyroradii larger than the width of TDs, they may pass upstream of TDs and generate FBs. Using multipoint observations from Advanced Composition Explorer, WIND, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), we report on two cases of TD‐driven FBs. The FBs were identified using classical FB selection criteria, and the driving TDs were identified using ideal MHD criteria, to within the limits of observational error. Our results add another potential solar wind driver of FBs and imply that FBs may be even more common than previously thought.

Several types of foreshock transients upstream of Earth's bow shock possessing a tenuous, hot core have been observed and simulated. Because of the low dynamic pressure in their cores, these phenomena can significantly disturb the bow shock and the magnetosphere‐ionosphere system. Recent observations have also demonstrated that foreshock transients can accelerate particles which, when transported earthward, can affect space weather. Understanding the potential of foreshock transients to accelerate particles can help us understand shock acceleration at Earth and at other planetary and astrophysical systems. To further investigate foreshock transients' potential for acceleration, we conduct a statistical study of ion and electron energization in the core of foreshock transients. We find that electron energies typically increase there, evidently due to an internal acceleration process, whereas, as expected, ion energies most often decrease to support transient formation and expansion. Nevertheless, ion energy enhancements can be seen in some events suggesting an internal ion acceleration process as well. Formation conditions of foreshock transients are related to weak solar wind magnetic field strength and fast solar wind speed. Ion and electron energization are also positively correlated with solar wind speed.